Many modern vehicles are equipped with advanced safety and driver-assist systems that require robust and precise object detection and tracking systems to control responsive host vehicle maneuvers. These systems utilize periodic or continuous detection of objects and control algorithms to estimate various object parameters, such as the relative object range, velocity, direction of travel, and size. For example, radar devices detect and locate objects (i.e., targets), by transmitting electromagnetic signals that reflect off targets within a sensor's field-of-view. The reflected signal returns to the radar as an echo where it is processed to determine various information such as the round-trip travel time of the transmitted/received energy. However, when multiple targets are present, certain radar devices lack the angular and spatial resolution necessary to distinguish between multiple targets that are closely-located (i.e., no point target assumption). In these cases, wherein two closely-located targets cannot be separated by range or angle, the targets may still be separated by a Doppler frequency if the Doppler resolution of the radar device is sufficiently high.
The Doppler effect manifests itself when there is a relative range rate, or radial velocity, between the radar and the target. When the radar's transmit signal is reflected from such a target, the carrier frequency of the return signal will be shifted. Assuming a collocated transmitter and receiver, the resulting Doppler frequency shift is a function of the carrier wavelength and the relative radial velocity (range rate) between the radar and the target. When the target is moving away from the radar, the relative radial velocity, or range rate, is defined to be positive and results in a negative Doppler shift.
Radar systems employing Doppler processing can be either continuous wave (CW) or pulsed. CW radars simply observe the Doppler shift between the carrier frequency of the return signal relative to the transmit signal. Pulse Doppler radars use a coherent train of pulses where there is a fixed or deterministic phase relationship of the carrier frequency between each successive radio frequency (RF) pulse. Coherence concentrates the energy in the frequency spectrum of the pulse train around distinct spectral lines, separated by the pulse repetition frequency (PRF). This separation into spectral lines allows for discrimination of Doppler shifts.
The pulsed nature of the transmitted signal permits time gating of the receiver, which allows for blanking of direct transmit energy leakage into the receiver. This permits the use of a single antenna for transmit and receive, which otherwise would not be feasible. Pulse Doppler radars can also use range gating, which divides the inter-pulse period into cells or range gates. The duration of each range gate is typically less than or equal to the inverse of the transmit pulse bandwidth. Range gating can help eliminate excess receiver noise from interfering with target return pulses, and allow range measurement with pulse delay ranging (i.e., measuring the time between transmission of a pulse and reception of the target echo).
The Doppler resolution of a radar is proportional to the Doppler processing integration time, which is the Doppler filter duration time. The Doppler processing integration time is typically determined by the duration that a reflection point (i.e., a target) remains within a range resolution cell. Conventionally, the Doppler processing integration time in radars is fixed across the Doppler frequency spectrum according to the highest known target speed for a particular radar application. Consequently, the Doppler resolution for these radars with respect to slower targets is limited.